Receivers in existing systems which provide terrestrial and/or satellite digital audio radio service (DARS) have been radically affected by blockage, shadowing and multipath effects which create severe degradations in signal quality, such as signal fading and inter-symbol interference (ISI) due to multipath. These effects on broadcast channels to receivers can be sensitive to location and frequency, particularly in an urban environment or geographic areas with high elevations where blockage of line of sight (LOS) signals from satellites is most prevalent.
Signal blockages at portable and mobile receivers can occur due to physical obstructions between a transmitter and the receiver. Mobile receivers, for example, encounter physical obstructions when they pass through tunnels or travel near buildings or trees that impede line of sight (LOS) satellite signal reception. Service outages can also occur when cancellations caused by multipath signal reflections are sufficiently high with respect to the desired signal.
Locations directly beneath a satellite (hereinafter referred to as the sub-satellite point) inherently have the highest LOS elevation angles, while locations that depart from the sub- satellite point inherently have decreasing LOS elevation angles and, accordingly, increasing likelihood of blockage and shadowing. Outdoor locations that are near the sub-satellite point typically enjoy virtually unblocked LOS reception. Thus, the need for terrestrial re-radiation of potentially blocked LOS signals is minimal. When the LOS elevation angle to the satellite becomes less than about 85 degrees, however, blockage by tall buildings or geological elevations (i.e., on the order of 30 meters) becomes significant. Terrestrial re-radiation for gap filling is needed to achieve satisfactory coverage for mobile radios, as well as static and portable radios. In areas where the heights of buildings or geological obstructions are relatively low (i.e., on the order of less than 10 meters), blockage is not significant until the distance from the sub-satellite point exceeds 1400 km, causing the LOS elevation angle to become lower than 75 degrees. For distances of 6300 km from the sub-satellite point, the elevation angle falls below 250 and the need for terrestrial re-radiation of the satellite signal increases significantly.
Thus, at the mid and high latitude locations within the coverages of one or more broadcast satellites, terrestrial re-radiation is needed to achieve suitable radio reception. For successful implementation of the reception by a mobile radio of direct LOS satellite signals and their combination with the same signals terrestrially re-radiated, near relative synchronization and combination of the satellite direct LOS signals with those repeated over the terrestrial network is needed at the receive site. Also, near synchronization among the signals re-radiated from the various terrestrial stations is needed at the receive sites.